[sdiy] SVF stable self oscillation (was OTA performance (was SSM chip reissue))

[David G Dixon]

I tried your suggestion in simulation, and it didn't work at all.  I tried
them in antiparallel across the feedback (that just caused the filter to hit
the rails), back-to-back (it again hit the rails after reaching the zener
amplitude), and back-to-back to ground (gave weird results).  Regular diodes
also hit the rails, or created a triangle oscillator in one configuration
(which may be useful, but is not what I'm after).

Yes, having back-to-back zeners across the BP cap does clip the BP output,
which does affect the LP and HP outputs slightly.  However, in my
simulations, it only affects the outcome at very high resonance, and only
clips the first sinusoid in the chain -- in essence, it limits the ringing
in much the same way that it limits the self-oscillation.  This is going to
have a very minor effect on the sound of the filter (it will ring ever so
slightly less than it otherwise would, which might actually be a good thing,
since I hate wailing filters).  I also don't see the amplitude effect in
simulation that I see in practice -- this could have to do with poorly
matched 2164 VCAs...?

So, I'm stuck with the conclusion that my technique is the only viable one
using zeners (which is basically the same conclusion I came to when I first
designed the uVCF, during which time I tried just about every configuration
of which I could conceive).
 

> -----Original Message-----
> From: Andrew Simper [mailto:andy at cytomic.com] 
> Sent: Wednesday, April 26, 2017 5:14 AM
> To: David G Dixon
> Cc: Neil Johnson; Andre Majorel; SDIY List
> Subject: SVF stable self oscillation (was OTA performance 
> (was SSM chip reissue))
> 
> Hi David,
> 
> Have you tried using the regular resonance limiting diode 
> setup for SVFs? The trouble with using clipping zeners at the 
> bandpass opamp buffer is you'll be clipping the bandpass 
> signal (taking current from the bandpass integrator cap) 
> which is then being sent to the low pass integrator, which 
> will introduce a frequency dependancy that won't give you a 
> nice sine wave at all cutoffs, I would expect you to lose 
> self oscillation as you lower the cutoff.
> 
> If you move the clipping to the feedback section then the two 
> integration stages are back in tune with each other and will 
> give the same Q no matter the cutoff. The regular way to do 
> this is by increasing the damping when the bandpass signal 
> louder with a pair of parallel but opposite direction diodes 
> in parallel to the damping feedback path.
> 
> As an aside the transposed (input mixing) SVF actually uses 
> the output signal to increase the damping, so you get a more 
> similar sound to an MS20 type sallen key, which also shapes 
> the output signal to control resonance.
> 
> Cheers,
> 
> Andy
> 
> 
> 
> 
> 
> On 26 April 2017 at 10:11, David G Dixon <dixon at mail.ubc.ca> wrote:
> > Yes, exactly.  A largish resistor between the BP output and 
> the input 
> > ensures robust oscillation at all frequencies.  Without 
> that resistor, 
> > just hitting the "theoretical" level of damping required for 
> > oscillation will not guarantee oscillation.
> >
> > A couple of zeners across the BP cap ensures relatively 
> clean sine waves.
> > However, I still haven't figured out how to get the sine 
> waves to have 
> > uniform amplitude at all frequencies.  One would think that 
> the zeners 
> > would ensure this, but they don't.  That's why, when I need really 
> > clean sine waves, I built a four-pole Roland filter and hardwire it 
> > for oscillation (with stage gains of 1.42 for an overall 
> loop gain of 
> > just slightly more than 4).  With back-to-back zeners off of the 
> > feedback, the output from the fourth stage gives very clean 
> (and very stable) sine waves.
> >
> >
> >> -----Original Message-----
> >> From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] 
> On Behalf Of 
> >> Andrew Simper
> >> Sent: Tuesday, April 25, 2017 6:11 PM
> >> To: Neil Johnson
> >> Cc: Andre Majorel; SDIY List
> >> Subject: Re: [sdiy] OTA performance (was SSM chip reissue)
> >>
> >> It by decent Q you mean Qs that match between voices sure 
> - this is 
> >> true for any filter topology. If by decent you mean high 
> values of Q 
> >> then for SVFs in synths you can get around this with positive 
> >> feedback around the bandpass integrator to push the damping more 
> >> negative and can easily push the circuit into self 
> oscillation, which 
> >> makes it just like any other topology where you can increase the 
> >> feedback past the point of self oscillation with more gain.
> >>
> >> Andy
> >>
> >> On 25 April 2017 at 18:30, Neil Johnson <neil.johnson71 at gmail.com> 
> >> wrote:
> >> >
> >> > Andre Majorel <aym-htnys at teaser.fr> wrote:
> >> > > How relevant gain matching within the same chip is to
> >> synth circuits
> >> > > is one of the many things I have no clue about. :-)
> >> >
> >> > It's also important in SVFs, where you want good gain
> >> matching between
> >> > the integrators if you want any decent Q.
> >> >
> >> > Neil
> >> > --
> >> > http://www.njohnson.co.uk
> >> > _______________________________________________
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